RR Aql Research Articles

Water vapour masers in long-period variable stars

Context. Water maser emission is often found in the circumstellar envelopes of evolved stars, that is, asymptotic giant branch stars and red supergiants with oxygen-rich chemistry. The H2O emission shows strong variability in evolved stars of both of these types. Aims. We wish to understand the reasons for the strong variability of water masers emitted at 22 GHz. In this paper, we study U Her and RR Aql as representatives of Mira variable stars. Methods. We monitored U Her and RR Aql in the 22 GHz maser line of water vapour with single-dish telescopes. The monitoring period covered about two decades between 1990 and 2011, with a gap between 1997 and 2000 in the case of RR Aql. Observations were also made in 1987 and 2015 before and after the period of contiguous monitoring. In addition, maps of U Her were obtained in the period 1990–1992 with the Very Large Array. Results. We find that the strongest emission in U Her is located in a shell with boundaries of 11–25 AU. The gas-crossing time is 8.5 yr. We derive lifetimes for individual maser clouds of ≤4 yr based on the absence of detectable line-of-sight velocity drifts of the maser emission. The shell is not evenly filled, and its structure is maintained over much longer timescales than those of individual maser clouds. Both stars show brightness variability on several timescales. The prevalent variation is periodic, following the optical variability of the stars with a lag of 2–3 months. Superposed are irregular fluctuations of a few months in duration, with increased or decreased excitation at particular locations, and long-term systematic variations on timescales of a decade or more. Conclusions. The properties of the maser emission are governed by those of the stellar wind while traversing the H2O maser shell. Inhomogeneities in the wind affecting the excitation conditions and prevalent beaming directions likely cause the variations seen on timescales of longer than the stellar pulsation period. We propose the existence of long-living regions in the shells, which maintain favourable excitation conditions on timescales of the wind-crossing times through the shells or orbital periods of (sub)stellar companions. The H2O maser properties in these two Mira variables are remarkably similar to those in the semiregular variables studied in our previous papers regarding shell location, outflow velocity, and lifetime. The only difference is the regular brightness variations of the Mira variables caused by the periodic pulsation of the stars.

A Very Long Baseline Array Trigonometric Parallax for RR Aql and the Mira Period–Luminosity Relation

We report Very Long Baseline Array observations of 22 GHz H2O and 43 GHz SiO masers toward the Mira variable RR Aql. By fitting the SiO maser emission to a circular ring, we estimate the absolute stellar position of RR Aql and find agreement with Gaia astrometry to within the joint uncertainty of ≈1 mas. Using the maser astrometry we measure a stellar parallax of 2.44 ± 0.07 mas, corresponding to a distance of pc. The maser parallax deviates significantly from the Gaia EDR3 parallax of 1.95 ± 0.11 mas, indicating a 3.8σ tension between radio and optical measurements. This tension is most likely caused by optical photocenter variations limiting the Gaia astrometric accuracy for this Mira variable. Combining infrared magnitudes with parallaxes for RR Aql and other Miras, we fit a period–luminosity relation using a Bayesian approach with Markov Chain Monte Carlo sampling and a strong prior for the slope of −3.60 ± 0.30 from the Large Magellanic Cloud. We find a K-band zero-point (defined at logP(days) = 2.30) of −6.79 ± 0.15 mag using very long baseline interferometry (VLBI) parallaxes and −7.08 ± 0.29 mag using Gaia parallaxes. The Gaia zero-point is statistically consistent with the more accurate VLBI value.

THE MEAN LIGHT CURVES OF THE MIRA-TYPE STARS IN THE H- AND K-BANDS

For nine Mira-type stars (o Cet, R Leo, S Car, U Her, X Oph, R Aql, RR Aql, S Ori, S Scl,) and one semi-regular star (L2 Pup), the mean light curves have been obtained. The initial values of the brightness (observations) were fitted by a trigonometric polynomial of the statistically optimal order. The Fourier coefficients and the additional parameters (degree of the trigonometric polynomial, amplitude of the brightness, the maximal slope of ascending and descending branch, semi-amplitudes and initial epochs for the brightness maximum (minimum magnitude) of each harmonic contribution, etc., were given in the tables reported earlier (Kudashkina, 2015). In this study, we have received several interesting correlations between these parameters in the bands H and K. It was particularly noted the anomalous position on the figures of the star X Oph. The mean light curves of the investigated stars are commonly symmetric in the near infrared region (H and K).

Mid-infrared interferometric observations of four oxygen-rich Mira variables

We present an investigation of the dust formation process in the atmospheres of the oxygen-rich AGB stars RR Aql, S Ori, GX Mon and R Cnc using spatially and spectrally resolved mid-infrared interferometric observations. We successfully compared multi epoch observations to a radiative transfer model of the dust shells, where the central stellar intensity profile is described by a series of dust-free dynamic model atmospheres based on self-excited pulsation models. We show that the Al 2 O 3 shells have inner radii between 1.9 and 2.2 stellar photospheric radii, and that the silicate shells have inner radii between 4.1 and 4.6 stellar photospheric radii. The best-fit photospheric angular diameters are consistent with independent estimates. The model dust temperatures at the inner radii of 1.9–2.2 stellar radii and 4.1–4.6 stellar radii are consistent with dust condensation temperatures of Al 2 O 3 and silicates, respectively.

Mid-infrared interferometric monitoring of evolved stars

Aims. We present a unique multi-epoch infrared interferometric study of the oxygen-rich Mira variable RR Aql in comparison to radiative transfer models of the dust shell. We investigate flux and visibility spectra at 8–13 μm with the aim of better understanding the pulsation mechanism and its connection to the dust condensation sequence and mass-loss process.

Improved VLBI astrometry of OH maser stars

Aims. Accurate distances to evolved stars with high mass loss rates are needed for studies of many of their fundamental properties. However, as these stars are heavily obscured and variable, optical and infrared astrometry is unable to provide enough accuracy. Methods. Astrometry using masers in the circumstellar envelopes can be used to overcome this problem. We have observed the OH masers of a number of Asymptotic Giant Branch (AGB) stars for approximately 1 year with the Very Long Baseline Array (VLBA). We have used the technique of phase referencing with in-beam calibrators to test the improvements this technique can provide to Very Long Baseline Interferometry (VLBI) OH maser astrometric observations. Results. We have significantly improved the parallax and proper motion measurements of the Mira variable stars U Her, S CrB and RR Aql. Conclusions. It is shown that both in-beam phase-referencing and a decrease in solar activity during the observations significantly improves the accuracy of the astrometric observations. The improved distances to S CrB ($418^{+21}_{-18}$ pc) and RR Aql ($633^{+214}_{-128}$ pc) are fully consistent with published P – L relations, but the distance to U Her ($266^{+32}_{-28}$ pc) is significantly smaller. We conclude that for sources that are bright and have a nearby in-beam calibrator, VLBI OH maser astrometry can be used to determine distances to OH masing stars of up to ~2 kpc.

Improving the parallaxes of OH bearing Miras

AbstractWe have carried out observations with the Very Long Baseline Array (VLBA) to measure the parallaxes of Mira variables and are able to improve the distance estimates significantly for a fraction of our sample (U Her, S CrB and RR Aql). This is predominantly because we have enhanced our technique by making use of nearby, in-beam calibrators. Additionally, the observing conditions have improved during the current solar minimum. The distances of these stars are of fundamental importance for studying the physical properties of Asymptotic Giant Branch (AGB) stars with high mass loss.

Long-term OH variability of Miras

We present here the results and interpretation of a long-term OH variability program conducted with the French Nancay Radiotelescope from 1980 to 1995. It concerns seven Mira stars: R Aql, RS Vir, S CrB, R LMi, RR Aql, U Her and UX Cyg. This study deals with the three OH maser lines observed in the Miras at 1612, 1665 and 1667 MHz. These OH variable stars have periods ranging from 290 to 580 days. The study presents the first insight of the long-term temporal behaviour of OH integrated flux variations as well as spectral component variations. The main aims are to determine the temporal behaviour of the OH maser emission and the longevity and variability of the spectral components. We find that the shapes of the OH curve are closer to the IR than the optical shapes and that the emissions at 1665 and 1667 MHz have a very similar behaviour while the emission at 1612 MHz behaves differently. The 1612 MHz emission shows smoother temporal variations and greater component longevity than the main line emission, leading to the conclusion that the 1612 MHz emission is coming from an outer part of the circumstellar shell and is more saturated than the main line emission. The study also shows the existence of inhomogeneities, especially differences between the front and back parts of the shell can be seen, and that OH variability curves undergo long term variations over several cycles.

Multiwavelength Studies of Mira Ceti-type Variable Stars

Since 1994, observations of a sample of about 20 Mira Ceti-type and semiregular variables have been carried out in three spectral ranges: radio (H2O maser line λ = 1.35 cm), optical (spectroscopy and UBV photometry) and infrared (JHKLM photometry). Time series of the Hα emission intensity and H2O line flux, covering several periods of the stars, have been obtained. Correlation of the intensity variations of the H2O maser with optical variability in the maser stars RR Aql, U Ori, VX Sgr and others was confirmed. One of the most interesting results is the flare of the H2O maser emission in R Leo, which happened in autumn 1997, 14 months after a flare of the Hα emission.

OH Maser Lines Variability of Miras

Up to now, OH variability studies of Miras dealt only with 2 or 3 consecutives cycles. Thus, changes in flux variations over longer times have never been studied. We present here the results of a long term variability high resolution program undertaken from 1980 to 1996 with the Nancay French radio telescope. It concerns 7 Miras: R Aql, RS Vir, S CrB, R LMi, RR Aql, U Her and UX Cyg. These stars cover the range of periods and infrared characteristics of Miras. The variability study presented here concerns the integrated flux as well as the cyclic variations of the spectral components with a longevity greater than one stellar period. These last ones proceed from a spectral decomposition program based on gaussian fitting (Etoka 1996). Such kind of long term variability study provides an insight into long term behaviour of circumstellar OH masers, showing interesting phenomena about the structure of the OH shell itself.

Envelope structures and optical/radio positions of cool stars

view Abstract Citations (83) References (85) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Envelope Structures and Optical/Radio Positions of Cool Stars Bowers, P. F. ; Johnston, K. J. ; de Vegt, C. Abstract Results are presented from 18-cm VLA observations of OH masers associated with the Mira variables IK Tau, U Her, R Aql, RR Aql, and the supergiant S Per. The linear radii of the OH shells range from about 35 AU to 450 AU. Evidence for axisymmetric structures are found for virtually all of the shells. A model to describe the observations is proposed in which material is flowing from the star in a radially expanding ellipsoidal configuration. It is concluded that axisymmetric outflow is common for red giants and supergiants and that the large magnetic field strength observed for these stars may influence the geometry and initiation of mass loss. Publication: The Astrophysical Journal Pub Date: May 1989 DOI: 10.1086/167411 Bibcode: 1989ApJ...340..479B Keywords: Cool Stars; Masers; Stellar Envelopes; Stellar Structure; Variable Stars; Astrometry; Astronomical Maps; Hydroxyl Emission; Position (Location); Radial Velocity; Stellar Mass Ejection; Very Large Array (Vla); Astrophysics; ASTROMETRY; INTERFEROMETRY; MASERS; STARS: CIRCUMSTELLAR SHELLS; STARS: LONG-PERIOD VARIABLES full text sources ADS | data products SIMBAD (12)

Investigation of a Sample of Long-Period Variable Stars Possessing Maser Emission

An investigation of parameters of a sample of more than two hundred late-type long-period variable stars (LPVs) has been carried out. In more detail were studied 13 giants (Mira Ceti-type variables R Aql, RR Aql, RT Aql, R Leo, U Ori, U Her, R Cas, R Tau, Z Cyg, R Peg, U Aur and semiregular variables RT Vir and RX Boo) and 2 supergiants (S Per, PZ Cas). A considerable fraction of the sample stars (about one third) possess circumstellar maser emission in molecular spectral lines (OH, H20, SiO). Our aim was to elucidate the particularities of photometric characteristics of maser stars, such as period P, amplitude A, light curve asymmetry f = (M-m)/P, and, in prospect, to determine their status in course of their evolution on the asymptotic giant branch. An extensive comparison with non-maser LPVs was made.

Studies of variability of circumstellar H2O masers

From March 1980 to December 1983, the author took part in regular observations of variability of maser radio emission in the H2O line at 22 GHz. The observations were carried out at the 22-meter radio telescope of the P. Lebedev Physical Institute (USSR Academy of Sciences) in Pushchino (Moscow Region). The interval between consecutive observational sessions was usually 1.5–2 months. The observational program included 21 late-type variable stars (Miras and SRs): R Aql, RR Aql, RT Aql, SY Aql, U Aur, NV Aur, RX Boo, VY CMa, S CrB, KY Cyg, NML Cyg, U Her, W Hya, X Hya, R Leo, U Lyn, U Ori, UU Peg, VX Sgr, RS Vir, RT Vir. The results for eight stars ending June 1982 were published by Berulis et alt (1983). A comparison was made between the time dependences of the H2O line radio flux F and the curves of visual and near-infrared brightness of the stars. Miras (R Aql, R Leo, U Ori, U Aur), as a rule, have a rise in F connected with the visual maximum (phase 0), the maximum F occurring at phases 0.1–0.2 (see figure for an example). Not all visual maxima (only one out of each two or three) are accompanied by H2O flares. This Miras! behaviour was also noted earlier in the H2O line by Berulis et al. (1984), Gómez Balboa and Lépine (1986), as well as in the SiO maser line v=1, J=2−1 by Nyman and Olofsson (1986).